The present invention relates to pulse columns used in solvent extraction processes, and more specifically, to an air pulser for use in regulating the air flow to a pulse leg connected to a pulse column, particularly as used in nuclear reactor fuel by-product extraction systems.
Solvent extraction is a conventional method used in reprocessing nuclear fuel. Once the spent fuels are dissolved into an aqueous solution, valuable by-products of the fission process can be recovered and separated from each other in significant quantities, and impurities may be reduced by many orders of magnitude.
The application of solvent extraction in nuclear fuel reprocessing involves a sequence of transfers of one or more solutes from a liquid phase to an essentially immiscible phase. The distribution of the solutes between the two phases is governed by principles of physical chemistry which are more or less well understood.
In such separation, the radioactive solutes ordinarily enter the system in an aqueous phase. At least some of the solutes are extracted into an organic phase, sometimes called the solvent. The organic or solvent phase may consist of a single substance, but frequently it contains one or more extractants and may include a diluent and sometimes a diluent modifier. The extracted solutes are subsequently removed from the organic phase by adjustment of chemical conditions such that stripping, also known as back extraction, occurs into an aqueous phase separate from the original feed stream.
Typical solvent extraction apparatus may be described as a series of interconnecting chambers in a linear arrangement or cascade. The aqueous phase is fed into the cascade at one end and the organic phase or the solvent is fed into the cascade at the opposite end. Thus, the aqueous phase and the organic phase move through the cascade in a continuous and counter-current flow pattern, with the aqueous and organic components interacting with each other in each chamber. In each chamber of the cascade, a portion of the desirable fission by-products is extracted into the solvent and thus removed from the aqueous phase. The cascade is designed so that the aqueous phase inlet and organic phase outlet are at the same end, and the aqueous phase outlet and the organic phase inlet are located together at the opposite end. At the aqueous phase outlet end, substantially all of the desirable products have been removed from the aqueous phase. Further, at the organic phase outlet end, the organic solvent is withdrawn from the cascade in a substantially loaded condition, with the desirable products contained therein. Subsequent chemical processing operations are used to further separate the fission products from the solvent solution.
Among the mechanisms generally used to practice solvent extraction is the pulse column. A pulse column is a liquid-liquid contactor having a cylindrical body or tower in which the rate of mass transfer is enhanced by hydraulic pulsation of the liquids in the column through a series of perforated plates.
In conventional pulse columns, a rapid reciprocating motion of relatively short amplitude is applied to the liquid contents of the column. An air pulser is normally employed to power this reciprocating motion and the consequential interaction of the aqueous and organic phases. Air pulse agitation has been found to give improved rates of extraction and to reduce tower heights compared to the dimensions of the former packed column apparatus. Consequently, when used in the processing of nuclear fuels and other radioactive chemical operations, the reduced size of pulse columns reduces the initial expense of installation as well as the cost of massive shielding.
Pulse columns also have a lower liquid retention capacity and therefor solvent degradation by radiation damage is decreased. Further, pulse columns provide a means of agitation not requiring moving parts, bearings and the like which come in contact with highly corrosive radioactive liquids. The amplitude and frequency of pulsations may be altered or varied depending on the application. Pulse frequency refers to the rate of application of the pulse action in terms of cycles/time, and pulse amplitude refers to the linear distance between extreme positions of the liquid in the column during pulsing.
In a conventional pulse column, a complex mechanical pulser assembly is provided, having an inlet valve and an exhaust valve. The assembly includes an electronic frequency controller to activate a four way solenoid valve which controls air flow to two-way pneumatic cylinders, one cylinder being provided for the inlet valve and one for the exhaust valve. Each cylinder is connected to a corresponding poppet valve, one each being respectively associated with the air inlet and exhaust conduits. In the first phase of the cycle, the assembly is adapted to open the air inlet valve and close the exhaust valve simultaneously, the increase in air pressure forcing liquid down the pulse leg. In the next phase of the pulse cycle, the inlet valve closes and the exhaust valve opens, venting the air from the pulse leg.
A significant disadvantage of the conventional air pulser assembly is inadequate venting of the pulse leg, which causes deviation from prescribed operational parameters. In addition, the conventional dual cylinder pulsing system is a relatively complex mechanical apparatus which suffers from reliability problems due to the substantial number of moving parts. Mechanical breakdowns have more impact when operating in the radioactive and corrosive environment of nuclear processing plants. Also, cycling efficiency suffers due to the interplay of mechanical parts. When the conventional air pulser system breaks down, the operation of the entire pulse column is suspended.
Thus, there is a need for a pulsing apparatus for use in a pulse column which has a minimum of operating parts and which is both reliable and accurate.